1. Field of Invention
The invention relates to the field of vortex flowmeters and, more particularly, to a design for a differential pressure transducer for use with a vortex flowmeter which requires no fill fluids, and which utilizes mechanical clamping of the vortex sensing elements.
2. Description of the Prior Art
The phenomenon of vortex shedding occurs over a certain Reynolds number range when a fluid (gas or liquid) flows past a bluff (non-streamlined) body. In a two-dimensional flow, the vortices formed on the opposite sides of the body rotate in an opposite sense from each other and form a regular geometrical pattern called the Karman vortex street. The convection velocity of this geometrical pattern is directly related solely to the approaching stream velocity. This means that the shedding frequency is proportional to the flow rate regardless of the fluid properties. The detection of number of the vortices shed per unit time (the vortex shedding frequency) and not its strength, is the prime measurement property of interest.
The role played by the interaction of the two shear layers on the sides of the bluff body is central in explaining the phenomenon of the alternation of vortex shedding. This was explained by J. H. Gerrard (1966) in Journal of Fluid Mechanics, 25, 401-413. A vortex spiral forms on either side of the wake of a bluff body or strut as a result of the shear layer instability. The vortex spiral continues to grow (see, for example, FIG. 4) until it is strong enough to draw the opposing shear layer across the near wake. The arrival of an oppositely signed vortex starts to weaken the ability of the spiral to draw vorticity from its connected shear layer. This allows the growing vortex to separate and move downstream. The bent shear layer from the other side that was sucked into the side of the spiral then begins to feed its own spiral on the opposite side of the strut. The presence of the strut between the two shear layers is essential for this alternation mechanism to work. This fact emphasizes the critical role played by the extent and shape of the back of the strut.
Vortex shedding results in alternating pressure depression on the two sides of the strut. A number of different techniques to detect the alternation frequency have been proposed. The cooling effect of the pressure depression or of a resulting induced flow that tries to restore the pressure balance is used in thermal sensors. Thermal sensors rely on having a heated element in the flow. This represents a potential hazard. The signal is often noisy and requires complicated electronic conditioning. The transducer element is delicate and subject to the aging process. The main advantage of thermal sensors is high sensitivity with a good time response.
In another technique. shuttle elements (discs, balls, etc.) are placed in a cavity that connects the two strut sides and move due to the induced flow that results from the pressure imbalance created by vortex shedding. Several proximity transducers to detect the movement of a shuttle element are available. The electronics are relatively simple when compared to thermal sensors. However, these techniques require passageways with openings that are susceptible to clogging and build-up of debris. The presence of a moving mechanical element limits the transducer time response and its fatigue cycle. The passageway is a leakage passage that affects the shedding process. In light of Gerrard's model (see above) that explains the alternation of vortex shedding, this leakage effect accelerates the equalization of the differential pressure, i.e. it increases the meter's K-factor (the number of pulses per total volume of fluid flowing through the meter). The leakage effect has a dependency on the fluid properties. In oscillating discs, thermal shocks can lead to serious distortions and damage to the disc.
In trying to eliminate the ports and the leakage passage used with shuttle elements, several proposals call for using diaphragms to seal the cavity of the shuttle element. A sealed shuttle element allows, in addition to proximity transducers, the use of a wider variety of strain transducers. Thin diaphragms are susceptible to thermal and pressure shocks. Often, an oil-filled cavity is used to support the diaphragms without serious damage to the response time of the sensor. Thermal expansion and phase change conditions limit the application range of an oil-filled cavity. In both the cases of an oil or atmospheric air filled cavity, a damaged diaphragm can lead to releasing the flow fluid to the outside environment. With few exceptions, such as shown in U.S. Patent No. 4,475,405, the replacement of the transducer requires depressurizing the flow line.
More recently, a proposal described in U.S. Patent No. 4,625,564 calls for using a fin in the passageway. The deflection of the fin activates strain transducers.
Other proposals describe techniques to measure the integrated pressure depressing along the side areas of the strut. This results in alternating lift and drag forces. The pivoting of the strut isolates either a bending force or a torsional torque from a shearing force. The integration of the stress along the strut increases the complexity of the design, since the noise generated anywhere along the strut needs to be eliminated. U.S. Patent No. 4,437,350 outlines the use of piezoelectric elements to sense the minute strains due to the micro-bending of the pivoted strut. The piezoelectric elements are embedded (e.g. cemented) within the strut itself. If the piezoelectric elements fails the whole strut must be replaced. The removal of the strut requires a flow shut down. The clamping of the element is not mechanical. This makes the arrangement more expensive and more susceptible to loss of signal if the cement softens, such as might occur at higher temperatures. U.S. Patent No. 4,699,012 describes a vortex sensing member disposed downstream of the vortex generator and parallel to it. The sensing member has a slender midsection to sense the lift forces downstream of the vortex generator. Different end support arrangements are proposed to suppress noise and amplify the vortex shedding effect.
In an alternative approach, the alternating drag forces are used to exert a micro-twist upon a torque tube located downstream of a primary shedder. The strains are transmitted through a link to piezoelectric sensing elements outside the flow.
There are other known techniques capable of detecting the vortices downstream of a shedder such as those which use ultrasonic transducers.
There is a need for an inexpensive and simple design for a differential pressure transducer for use with a vortex flowmeter. Local measurements rather than integrated ones are desirable since they tend to be less noisy. A mechanically clamped transducer design is simpler and more serviceable than welded or cemented ones. A non-welded sensor that can be replaced without depressurizing the flow line possesses a clear advantage. The elimination of all leakage passages and ports shown in some types of prior art devices not only would avoid clogging of these passages with debris, but it also would result in a more linear output independent of the fluid properties. Another important property of a sensor for a vortex flowmeter is the ability to reject common-mode noise, i.e. vibrations due to sources other than the alternating vortices shed by the bluff body or strut.